LIFePO4 FLAKES FOR LI-ION BATTERY AND METHOD FOR MANUFACTURING THE SAME

ABSTRACT

LiFePO 4  flakes for a Li-ion battery and a method for manufacturing the same are disclosed. The LiFePO 4  flakes of the present invention have a thickness of 5 nm-200 nm, and the angle between the flat surface normal of the flake and the Li-ion diffusion channel is 0°-80°. In addition, according to the present invention, the LiFePO 4  flakes with short Li ion diffusion path can be prepared through a simple process. Hence, not only the charge-discharge efficiency of the Li-ion battery can be improved by use of the LiFePO 4  flakes of the present invention, but also the cost of the Li-ion battery can be further reduced.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/529,221, filed on Jun. 21, 2012. This application claims thebenefits of the Taiwan Patent Application Serial Number 100121791, filedon Jun. 22, 2011, the subject matter of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to LiFePO₄ flakes and a method formanufacturing the same, more particularly, to LiFePO₄ flakes for aLi-ion battery and a method for manufacturing the same.

2. Description of Related Art

As the development of various portable electronic devices continues,more and more attention focuses on the techniques of energy storage, andbatteries are the main power supplies for these portable electronicdevices. Among commercial batteries, small-sized secondary batteries areespecially the major power supplies for portable electronic devices suchas cell phones and notebooks. In addition, secondary batteries areapplied to not only portable electronic devices, but also electricvehicles.

Among the developed secondary batteries, the lithium secondary batteries(also named as the Li-ion batteries) developed in 1990 are the mostpopular batteries used nowadays. The cathode material of the initiallithium secondary batteries is LiCoO₂. LiCoO₂ has the properties of highworking voltage and stable charging and discharging voltage, so thesecondary batteries which use LiCoO₂ as a cathode material are widelyapplied to portable electronic devices. Then, LiFePO₄ with an olivinestructure and LiMn₂O₄ with a spinal structure were also developed as acathode material for lithium secondary batteries. Compared to LiCoO₂,the safety of the batteries can be improved, the charge/discharge cyclescan be increased, and the cost can be further reduced when LiFePO4 orLiMn₂O₄ is used as cathode material of secondary batteries.

Although the batteries which use LiMn₂O₄ as cathode materials have lowcost and improved safety, the spinal structure of LiMn₂O₄ may collapseduring the deep discharge process, due to Jahn-Teller effect. In thiscase, the cycle performance of the batteries may further degrade. WhenLiFePO₄ is used as cathode material of batteries, the batteries alsohave the properties of low cost and improved safety. In addition, thecapacity of LiFePO₄ is higher than that of LiMn₂O₄, so the batteriesmade from LiFePO₄ can further be applied to devices, which need largecurrent and high power. Furthermore, LiFePO₄ is a non-toxic andenvironmentally friendly material, and also has great high temperaturecharacteristics. Hence, LiFePO₄ is considered as an excellent cathodematerial for lithium batteries. Currently, the average discharge voltageof the lithium batteries using LiFePO₄ as a cathode material is 3.4˜3.7V vs. Li⁺/Li.

A conventional structure of the Li-ion batteries comprises: a cathode,an anode, a separator, and a Li-containing electrolyte. The batteriesperform the charge/discharge cycles by the lithium insertion andextraction mechanism, which is represented by the following equations(I) and (II).

Charge: LiFePO₄ −xLi⁺ −xe ⁻ →xFePO₄+(1-x)LiFePO₄  (I)

Discharge: FePO₄ +x Li⁺ +xe ⁻ →x LiFePO₄+(1-x)FePO₄  (II)

When a charge process of the batteries is performed, Li ions extractfrom the structure of LiFePO₄; and the Li ions insert into the structureof FePO₄ when a discharge process is performed. Hence, thecharge/discharge process of the Li-ion batteries is a two-phase processof LiFePO₄/FePO₄. However, the diffusion rate of the Li ions in LiFePO₄and FePO₄ is quite low, so some metal dopants may generally be addedinto the cathode materials to increase the diffusion rate of the Liions. In addition, the Li-ion diffusion path of the currently usedLiFePO₄ is long, so the conductivity of LiFePO₄ is not good enough.Hence, the LiFePO₄ powders may be coated with carbon to increase theconductivity of LiFePO₄ powders, in order to increase thecharge/discharge efficiency of the batteries. However, the process formanufacturing LiFePO₄ powders becomes more complicated when the LiFePO₄powders are doped with metal additives or coated with carbon. In thiscase, the cost of the batteries is also increased.

Currently, the LiFePO₄ powders are usually prepared by a solid-stateprocess. However, the property of the product is highly related to thesintering temperature of the solid-state process. When the sinteringtemperature is below 700° C., all the raw materials have to be mixedwell. If the raw materials are not mixed well, Fe³⁺ impurity phase willbe present in the LiFePO₄ powders. When sintering temperature is below600° C., the average grain size of the LiFePO₄ powders will be smallerthan 30 μm. However, if the sintering temperature is increased, theaverage grain size of the LiFePO₄ powders will be larger than 30 μm.When the average grain size of the LiFePO₄ powders is larger than 30 μm,a grinding process and a sieving process have to be performed to obtainpowders with grain size between 1 μm to 10 μm. Hence, it is difficult toprepare LiFePO₄ powders with nano-size by the use of the solid-stateprocess.

Therefore, it is desirable to provide a method for manufacturingnano-sized cathode materials of Li-ion batteries in a simple way, inorder to increase the charge/discharge efficiency of the batteries andreduce the cost thereof.

SUMMARY OF THE INVENTION

The object of the present invention is to provide LiFePO₄ flakes with ashort (Li-ion) diffusion path for use in Li-ion batteries. Hence, thecharge/discharge efficiency of the Li-ion battery can further beincreased.

Another object of the present invention is to provide a method formanufacturing LiFePO₄ flakes for a Li-ion battery. LiFePO₄ flakes with ashort Li-ion diffusion path can be obtained through a simple process byusing the method of the present invention, so the cost of the Li-ionbatteries can be reduced.

To achieve the object, the present invention provides LiFePO₄ flakes,which have flat shapes, for use in a Li-ion battery. In addition, thethickness of the LiFePO₄ flakes is 5 nm-200 nm, and an angle between aflat surface normal of the LiFePO₄ flakes and a Li-ion diffusion channelis 0°-80°. According to the present invention, the term “normal” meansthe line, which is perpendicular to the flat surface of the LiFePO₄flakes.

In addition, the present invention also provides a method formanufacturing the aforementioned LiFePO₄ flakes, which comprises thefollowing steps: (A) providing a mixed organic solution which comprisesLi, Fe, and P, wherein the Li contained in the mixed organic solution isderived from a Li-containing precursor or a P and Li-containingprecursor, the Fe contained in the mixed organic solution is derivedfrom an Fe-containing precursor or a P and Fe-containing precursor, andthe P contained in the mixed solution is derived from a P-containingprecursor, a P and Li-containing precursor, or a P and Fe-containingprecursor; (B) heating the organic mixed solution to obtain preliminaryproducts; and (C) heat-treating the preliminary products to obtainLiFePO₄ flakes.

According to the LiFePO₄ flakes for the Li-ion battery and the methodfor manufacturing the same of the present invention, precursorscontained in the organic mixed solution are reacted with each other, andthen the LiFePO₄ flakes can further be obtained after heat treatment. Inaddition, the obtained LiFePO₄ flakes of the present invention have anordered crystal lattice and a short Li-ion diffusion path. The obtainedLiFePO₄ flakes of the present invention have flat shapes, evenpetal-like shapes, so the thickness of the flakes is very thin.Therefore, the insertion and extraction path of the Li ions can begreatly shortened, and the diffusion rate of the Li ions can further beincreased. When the obtained LiFePO₄ flakes of the present invention areused as cathode materials of a Li-ion battery, the charge-dischargeefficiency of the Li-ion battery can be increased due to the shortLi-ion diffusion path. In addition, the efficiency of the Li-ion batterycan be improved by using the LiFePO₄ flakes of the present inventionwithout doping any additives or coating any conductive materials, so thecost for manufacturing the Li-ion batteries can be further reduced.

According to the LiFePO₄ flakes for the Li-ion battery of the presentinvention, an angle between a zone axis of the flat surface of theLiFePO₄ flakes and a b-axis [0, 1, 0] of the Li-ion diffusion channel is0°-80°, so an angle between a flat surface normal of the LiFePO₄ flakesand a Li-ion diffusion channel is 0°-80°. Preferably, the angle betweenthe zone axis of the flat surface of the LiFePO₄ flakes and the b-axis[0, 1, 0] of the Li-ion diffusion channel is 30°-65°, i.e. the anglebetween the flat surface normal of the LiFePO₄ flakes and the Li-iondiffusion channel is 30°-65°. More preferably, the angle between thezone axis of the flat surface of the LiFePO₄ flakes and the b-axis [0,1, 0] of the Li-ion diffusion channel is 45°-60°, i.e. the angle betweenthe flat surface normal of the LiFePO₄ flakes and the Li-ion diffusionchannel is 45°-60°.

Preferably, the zone axis of the flat surface of the LiFePO₄ flakes isclose to [1, −3, 5].

In addition, according to the LiFePO₄ flakes for the Li-ion battery ofthe present invention, the width of the LiFePO₄ flakes is notparticularly limited. Preferably, the width of the LiFePO₄ flakes is 500nm-5 μm. More preferably, the width of the LiFePO₄ flakes is 1 μm-2.5μm.

Furthermore, according to the LiFePO₄ flakes for the Li-ion battery ofthe present invention, the LiFePO₄ flakes have olivine structures.

According to the method for manufacturing the LiFePO₄ flakes of thepresent invention, a small amount of metal-containing compounds may befurther added in the mixed organic solution in step (A), and the dopedmetal in the obtained LiFePO₄ flakes can increase the conductivitythereof. Herein, the doped metal can be at least one selected from thegroup consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Zr, Tc, Ru, Rh,Pd, Ag, Cd, Pt, Au, Al, Ga, In, Be, Mg, Ca, Sr, B, and Nb. Preferably,the metal-containing compounds are sulfates, carbonates, nitrates,oxalates, acetates, chlorites, bromides, or iodides of theaforementioned doped metals. More preferably, the metal-containingcompounds are sulfates of the aforementioned doped metals. Mostpreferably, the metal-containing compounds are sulfates of Mn, Cr, Co,Cu, Ni, Zn, Al, or Mg.

Hence, the obtained LiFePO₄ flakes of the present invention arerepresented by the following formula (I):

LiFe_((1-x))M_(x)PO₄  (I)

wherein, M can be one or more metals selected from the group consistingof Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Zr, Tc, Ru, Rh, Pd, Ag, Cd, Pt,Au, Al, Ga, In, Be, Mg, Ca, Sr, B, and Nb, and 0≦x<0.5. Preferably, M isone or more metals selected from the group consisting of Mn, Cr, Co, Cu,Ni, Zn, Al, and Mg. More preferably, M is one or more metals selectedfrom the group consisting of Mn, Cu, Zn, Al, Ni, and Mg.

In addition, according to the method for manufacturing the LiFePO₄flakes of the present invention, a small amount of carbon material maybe further added in the mixed organic solution in step (A). In thiscase, the surfaces of the obtained LiFePO₄ flakes are coated withcarbon, so the conductivity of the obtained LiFePO₄ flakes can furtherbe increased. Herein, the carbon materials can be any sugar, such assucrose. In addition, the additive amount of the carbon material can be0.1 wt %-20 wt % of the weight of the obtained LiFePO₄ flakes.

According to the method for manufacturing the LiFePO₄ flakes of thepresent invention, the Li-containing precursor can be at least oneselected from the group consisting of LiOH, Li₂CO₃, LiNO₃, CH₃COOLi,Li₂C₂O₄, Li₂SO₄, LiCl, LiBr, and LiI; the Fe-containing precursor can beat least one selected from the group consisting of FeCl₂, FeBr₂, FeI₂,FeSO₄, (NH₄)₂Fe(SO₄)₂, Fe(NO₃)₂, FeC₂O₄, (CH₃COO)₂Fe, and FeCO₃; theP-containing precursor can be at least one selected from the groupconsisting of H₃PO₄, NaH₂PO₄, Na₂HPO₄, Mg₃(PO₄)₂, and NH₄H₂PO₄; the Pand Li-containing precursor can be at least one selected from the groupconsisting of LiH₂PO₄, Li₂HPO₄, and Li₃PO₄; and the P and Fe-containingprecursor can be at least one selected from the group consisting ofFe₃(PO₄)₂, and FePO₄. According to the method for manufacturing theLiFePO₄ flakes of the present invention, the organic mixed solution notonly contains the aforementioned precursors, but also may contain otheradditives such as surfactants, polymer electrolytes, and stabilizers,which can facilitate the synthesis reaction or the dissolution of theprecursors. Herein, the examples of the surfactants can beCetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate(SDBS), sodium dodecyl sulfate (SDS), or Triton-X100; the examples ofthe polymer electrolytes can be polyvinylpyrollidone (PVP), peroxyaceticacid (PAA), Polyethyleneimine (PEI), or polyacrylamide (PAM); and theexamples of the stabilizers can be Polyvinyl Alcohol (PVA), or polyvinylacetate (PVAc). The aforementioned additives can control the crystalsize and the preferential growth direction of the powders. In addition,in step (B) of the method of the present invention, an external electricfield can be further applied to control the preferential growthdirection of the crystal of the powders, in order to increase the areaof the Li-ion transfer interface.

According to the method for manufacturing the LiFePO₄ flakes of thepresent invention, the organic solvent in the mixed organic solution canbe any media generally used in the art, and can be any polyol organicsolvents generally used in the art. Preferably, the organic solvent isat least one selected from the group consisting of ethylene glycol (EG),diethylene glycol (DEG), glycerol, triethylene glycol (TEG),tetraethylene glycol (TTEG), polyethylene glycol (PEG), Dimethylsulfoxide (DMSO), and N,N-dimethylmethanamide (DMF). More preferably,the organic solvent used in the method of the present invention is DEG,glycerol, or a combination thereof.

Furthermore, according to the method for manufacturing the LiFePO₄flakes of the present invention, preferably, the organic mixed solutionis reacted by reflux. Herein, the reaction temperature of reflux is 105°C.-350° C., and the reaction thereof is 10 hrs-20 hrs, preferably.

In addition, according to the method for manufacturing the LiFePO₄flakes of the present invention, the organic mixed solution may beheated and reacted under an atmosphere or with an introduced gas flow toobtain the preliminary products in step (B). Herein, the atmosphere orthe introduced gas flow can be used as a protective gas or a reducinggas, which may comprise one selected from the group consisting of N₂,H₂, He, Ne, Ar, Kr, Xe, CO, methane, N₂—H₂ mixture, and a mixturethereof. Preferably, the protective gas or the reducing gas is N₂, H₂,or N₂—H₂ mixture. More preferably, the protective gas or the reducinggas is N₂—H₂ mixture.

According to the method for manufacturing the LiFePO₄ flakes of thepresent invention, the preliminary products may be heat-treated under anatmosphere or with an introduced gas flow to obtain the LiFePO₄ flakesin step (C). Herein, the atmosphere or the introduced gas flow can alsobe used as a protective gas or a reducing gas, which may comprise oneselected from the group consisting of N₂, H₂, He, Ne, Ar, Kr, Xe, CO,methane, N₂—H₂ mixture, and a mixture thereof. Preferably, theprotective gas or the reducing gas is N₂, H₂, or N₂—H₂ mixture. Morepreferably, the protective gas or the reducing gas is N₂—H₂ mixture.

In step (C) of the method for manufacturing the LiFePO₄ flakes of thepresent invention, the preliminary products are heat-treated at 300°C.-700° C., preferably. In addition, the preliminary products arepreferably heat-treated for 1 hr-20 hrs.

In addition, the obtained LiFePO₄ flakes of the present invention can beused as a cathode material to prepare a Li-ion battery, through anyconventional method in the art. Here, the method for manufacturing theLi-ion battery is briefly described, but the present invention is notlimited thereto.

An anode current collector is coated with a carbon material, and thenthe carbon material is dried and pressed to form an anode for the Li-ionbattery. A cathode current collector is coated with a cathode activematerial (i.e. the LiFePO₄ of the present invention), and then thecathode active material is dried and pressed to form a cathode for theLi-ion battery. Next, a separator is inserted between the cathode andthe anode, a Li-containing electrolyte is injected, and a Li-ion batteryis obtained after packaging.

Other objects, advantages, and novel features of the invention willbecome more apparent from the following detailed description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an XRD diffraction pattern of preliminary products beforeheat-treating process according to Embodiment 1 of the presentinvention;

FIG. 1B is an XRD diffraction pattern of LiFePO₄ flakes according toEmbodiment 1 of the present invention;

FIGS. 2A and 2B are SEM photos of LiFePO₄ flakes according to Embodiment1 of the present invention;

FIG. 3 is a schematic drawing of the Li ion diffusion path of LiFePO₄flakes according to Embodiment 1 of the present invention; and

FIG. 4 is SEM photo of LiFePO₄ flakes according to Embodiment 3 of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention has been described in an illustrative manner, andit is to be understood that the terminology used is intended to be inthe nature of description rather than of limitation. Many modificationsand variations of the present invention are possible in light of theabove teachings. Therefore, it is to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

Embodiment 1

The precursors of H₃PO₄, FeC₂O₄, and LiOH were mixed in a ratio of1:1:1, and dissolved in DEG (300 ml) to obtain a mixed organic solution(0.15 M). Next, the mixed organic solution was reacted by reflux at 200°C., under an atmosphere of N₂—H₂ mixture for 18 hrs. After the mixedorganic solution was filtrated, preliminary products were obtained.Then, the preliminary products were heat-treated at 500° C., under anatmosphere of N₂ gas for 2 hrs to obtain LiFePO₄ flakes of the presentembodiment.

The preliminary products before heat-treatment of the present embodimentwere examined by an X-ray diffractometer (Shimadzu 6000) to obtain thecrystal structure thereof. The X-ray diffraction pattern was obtained byapplying Cu Kα radiation, the 2θ-scanning angle is 15°-45°, and thescanning rate is 1°/min. The obtained XRD diffraction patterns are shownin FIGS. 1A and 1B, wherein FIG. 1A is an XRD diffraction pattern ofpreliminary products before heat-treatment, and FIG. 1B is an XRDdiffraction pattern of LiFePO₄ flakes.

According to FIGS. 1A and 1B, the upper figures are the XRD diffractionpatterns of the preliminary products and the LiFePO₄ flakes of thepresent embodiment, and the lower figures are the XRD diffractionpatterns of LiFePO₄ crystal with an olivine structure (JCPDS No.81-1173). As shown in the upper figures of FIGS. 1A and 1B, lines areused to indicate the position of the peaks.

As shown in FIG. 1A, the preliminary products before heat-treatment donot have olivine structures. On the contrary, as shown in FIG. 1B, thelines indicating the peaks of the LiFePO₄ flakes of the presentembodiment correspond to the peaks of LiFePO₄ crystal with the olivinestructure (JCPDS No. 81-1173), and there are no other peaks observed inthe LiFePO₄ flakes of the present embodiment. This result indicates thatall the LiFePO₄ flakes of the present embodiment are LiFePO₄ flakes witholivine structures.

In addition, the shapes of the LiFePO₄ flakes of the present embodimentwere observed with a scanning electron microscope (SEM) (HitachiS-4000), and the results are shown in FIGS. 2A and 2B. As shown in FIG.2A, the LiFePO₄ flakes have flat shapes (also named as petal-likeshapes), and the width of each flake is 2 μm, when the LiFePO₄ flakeswere observed at the magnification of 100000×. As shown in FIG. 2B, whenthe LiFePO₄ flakes were observed at the magnification of 600000×, it canbe observed that the thickness of each flake is 50 nm, and the width ofthe surface of each flake is 40 folds of the thickness of each flake.

Furthermore, a transmission electron microscope (TEM) (JEOL 2010) wasused to observe the single-crystal diffraction of the powders. Theresult shows that the zone axis of the LiFePO₄ flakes of the presentembodiment is close to [1, −3, 5], and the angle between the zone axis[1, −3, 5] and the surface normal of each diffraction planes is in arange from 89.33° to 90.33°. After calculation, the angle between thezone axis [1, −3, 5] and a b-axis [0, 1, 0] of the Li-ion diffusionchannel is 54.93°, which is obtained by the following equations:

[1, −3, 5] ⋅ [0, 1, 0] = [1, −3, 5] × [0, 1, 0] × cos  θ${\cos \; \theta} = {\frac{{- 3}b^{2}}{\sqrt{a^{2} + {9b^{2}} + {25c^{2}}} \times \sqrt{b^{2}}} = {- 0.5753}}$

wherein, a is 10.33 {acute over (Å)}, b is 6.01 {acute over (Å)}, c is4.692 {acute over (Å)}, and a, b and c are respectively the length ofthe unit vector along the x, y and z crystal axes of the LiFePO₄ flakes.After calculation, θ is 125.07°, which can be taken into) 54.93°(180°-125.07°=54.93°.

In other words, as shown in FIG. 3, the angle α between the Li-iondiffusion channel and the normal of the surface 101 of the flake of theLiFePO₄ flakes 10 of the present embodiment is 54.93°. The longestdiffusion path x for the Li-ion to leave the LiFePO₄ flakes with thethickness D of 50 nm is (50 nm/(2×cos 54.93°))=43.5 nm.

Embodiment 2

The precursors of H₃PO₄, FeSO₄, and Li₂CO₃ (1:1:1) and urea with thesame molar amount were dissolved in DEG (300 ml) to obtain a mixedorganic solution (0.3 M). Next, the mixed organic solution was reactedby reflux at 200° C., under an atmosphere of N₂—H₂ mixture for 14 hrs.After the mixed organic solution was filtrated, preliminary productswere obtained. Then, the preliminary products were heat-treated at 500°C., under an atmosphere of N₂ gas for 10 hrs to obtain LiFePO₄ flakes ofthe present embodiment.

The LiFePO₄ flakes of the present embodiment were examined with an X-raydiffractometer, a scanning electron microscope, and a transmissionelectron microscope by the same methods as disclosed in Embodiment 1.The result of the X-ray diffraction shows that the LiFePO₄ flakes of thepresent embodiment have olivine structures. The result of SEM shows thatthe LiFePO₄ flakes of the present embodiment have flat shapes, the widthof each flake is 1-1.5 μm, the thickness thereof is 20-35 nm, and thewidth of the surface is 43-50 folds of the thickness thereof.

In addition, the result of TEM shows that the angle between the zoneaxis and the surface normal of each diffraction planes of the LiFePO₄flakes of the present embodiment is in a range from 89.33° to 90.33°.The angle between the zone axis [1, −3, 5] and the b-axis [0, 1, 0] ofthe Li-ion diffusion channel is calculated to be 54°.

Embodiment 3

The precursors of H₃PO₄, FeC₂O₄, and LiOH were mixed with a ratio of1:1:1, and dissolved in DEG (300 ml) to obtain a mixed organic solution(0.15 M). Next, SDS was added into the mixed organic solution, and themixed organic solution was reacted by reflux at 200° C., under anatmosphere of N₂ gas for 10 hrs. After the mixed organic solution wasfiltrated, preliminary products were obtained. Then, the preliminaryproducts were heat-treated at 500° C., under an atmosphere of N₂ gas for4 hrs to obtain LiFePO₄ flakes of the present embodiment.

The shapes of the LiFePO₄ flakes of the present embodiment were observedwith a scanning electron microscope (SEM). As shown in FIG. 4, theLiFePO₄ flakes have flat shapes (also named as petal-like shape) andbundle like a rose, and the thickness of each flake is thin and uniform,when the LiFePO₄ flakes were observed at the magnification of 100000×.In addition, the SEM result also shows that the dispersion of theLiFePO₄ flakes of the present embodiment is better than that ofEmbodiment 1.

According to the results of Embodiments 1 to 3, the LiFePO₄ flakes ofthe present invention have unique shapes, so the distribution density ofthe Li-ion diffusion channel is the highest. In addition, the LiFePO₄flakes of the present invention are flakes, so Li ions can extract fromthe powders in a uniform and high-density manner Therefore, the currentdensity of the Li-ion batteries can be further increased by the use ofthe LiFePO₄ flakes of the present invention.

Furthermore, the length of the Li-ion diffusion channel is similar tothe thickness of the LiFePO₄ flakes of the present invention, so Li ionscan quickly diffuse inside the powders. Even though there are no metaladditives doped into the LiFePO₄ flakes of the present invention, thehigh diffusion rate of the Li ions can still be maintained. In addition,the number of Li ions extracting from the powders is not significantlyreduced over time, so an almost constant charge/discharge current can beobtained. Therefore, even if the charge/discharge time is increased, theproblem of the reduction of charge/discharge current can be prevented bythe use of the LiFePO₄ flakes of the present invention.

In addition, the length of the Li-ion diffusion channel according to theLiFePO₄ flakes of the present invention is in nano scale, so the problemof low conductivity can be avoided. Hence, when the LiFePO₄ flakes ofthe present invention are applied to Li-ion batteries, cathode materialswith high conductivity can be obtained without carbon coating.

In conclusion, according to the LiFePO₄ flakes and the method formanufacturing the same of the present invention, the thickness of theobtained LiFePO₄ flakes is only of nano scale. Even though there are nometal additives doped into the LiFePO₄ flakes or no carbon coated on thesurface of the LiFePO₄ flakes, the charge/discharge character of thebatteries can still be improved. In addition, the process of the methodfor manufacturing the LiFePO₄ flakes of the present invention is simple.Therefore, when the LiFePO₄ flakes of the present invention are appliedto Li-ion batteries, the cost thereof can be further decreased.

Although the present invention has been explained by its preferredembodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

What is claimed is:
 1. A method for manufacturing LiFePO₄ flakes for aLi-ion battery, comprising the following steps: (A) providing a mixedorganic solution by reflux, which comprises Li, Fe, and P, wherein theLi contained in the mixed organic solution is derived from aLi-containing precursor or a P and Li-containing precursor, the Fecontained in the mixed organic solution is derived from an Fe-containingprecursor or a P and Fe-containing precursor, and the P contained in themixed solution is derived from a P-containing precursor, a P andLi-containing precursor, or a P and Fe-containing precursor; (B) heatingthe organic mixed solution to obtain preliminary products; and (C)heat-treating the preliminary products to obtain LiFePO₄ flakes.
 2. Themethod as claimed in claim 1, wherein the organic mixed solution isheated under an atmosphere or with an introduced gas flow to obtain thepreliminary products, in step (B).
 3. The method as claimed in claim 1,wherein the preliminary products are heat-treated under an atmosphere orwith an introduced gas flow to obtain the LiFePO₄ flakes, in step (C).4. The method as claimed in claim 1, wherein the Li-containing precursoris at least one selected from the group consisting of LiOH, Li₂CO₃,LiNO₃, CH₃COOLi, Li₂C₂O₄, Li₂SO₄, LiCl, LiBr, and LiI.
 5. The method asclaimed in claim 1, wherein the Fe-containing precursor is at least oneselected from the group consisting of FeCl₂, FeBr₂, FeI₂, FeSO₄,(NH₄)₂Fe(SO₄)₂, Fe(NO₃)₂, FeC₂O₄, (CH₃COO)₂Fe, and FeCO₃.
 6. The methodas claimed in claim 1, wherein the P-containing precursor is at leastone selected from the group consisting of H₃PO₄, NaH₂PO₄, Na₂HPO₄,Mg₃(PO₄)₂, and NH₄H₂PO₄.
 7. The method as claimed in claim 1, whereinthe P and Li-containing precursor is at least one selected from thegroup consisting of LiH₂PO₄, Li₂HPO₄, and Li₃PO₄.
 8. The method asclaimed in claim 1, wherein the P and Fe-containing precursor is atleast one selected from the group consisting of Fe₃(PO₄)₂, and FePO₄. 9.The method as claimed in claim 1, wherein the mixed organic solutionfurther comprises: at least one selected from the group consisting of asurfactant, a polymer electrolyte, and a stabilizer.
 10. The method asclaimed in claim 1, wherein an organic solvent in the mixed organicsolution is at least one selected from the group consisting of ethyleneglycol (EG), diethylene glycol (DEG), glycerol, triethylene glycol(TEG), tetraethylene glycol (TTEG), polyethylene glycol (PEG), Dimethylsulfoxide (DMSO), and N,N-dimethylmethanamide (DMF).
 11. The method asclaimed in claim 2, wherein the atmosphere or the introduced gas flowcomprises one selected from the group consisting of N₂, He, Ne, Ar, Kr,Xe, CO, methane, N₂—H₂ mixed gas, and a mixture thereof.
 12. The methodas claimed in claim 3, wherein the atmosphere or the introduced gas flowcomprises one selected from the group consisting of N₂, He, Ne, Ar, Kr,Xe, CO, methane, N₂—H₂ mixed gas, and a mixture thereof.
 13. The methodas claimed in claim 9, wherein the surfactant is Cetyltrimethylammoniumbromide (CTAB), sodium dodecylbenzene sulfonate (SDBS), sodium dodecylsulfate (SDS), or Triton-X100.
 14. The method as claimed in claim 9,wherein the polymer electrolyte is polyvinylpyrollidone (PVP),peroxyacetic acid (PAA), Polyethyleneimine (PEI), or polyacrylamide(PAM).
 15. The method as claimed in claim 9, wherein the stabilizer isPolyvinyl Alcohol (PVA), or polyvinyl acetate (PVAc).
 16. The method asclaimed in claim 1, wherein the organic mixed solution is heated to andreacted at 105°-350° C., in step (B).
 17. The method as claimed in claim1, wherein the organic mixed solution is reacted for 10 hrs-20 hrs. 18.The method as claimed in claim 1, wherein the preliminary products areheat-treated at 300°-700° C., in step (C).
 19. The method as claimed inclaim 1, wherein the preliminary products are heat-treated for 1 hr-20hrs.
 20. The method as claimed in claim 1, wherein the LiFePO₄ flakeshave a thickness of 5 nm-200 nm, and an angle between a flat surfacenormal of the LiFePO₄ flakes and a Li-ion diffusion channel is 0°-80°.21. The method as claimed in claim 1, wherein the LiFePO₄ flakes haveolivine structures.